The Effect of Defects on the Laser Damage Performance of Metal Mirror Surfaces

From a laser damage standpoint, the optical surface is not isotropic as evidenced by the selected area damage phenomenon. This is because the modern processes that generate specular surfaces and coatings inherently introduce defects and damage into the bulk material. These defects then act as preferential sites for the onset of laser damage, and the distribution of these defects appears to affect almost all laser damage parameters. This paper presents the development of high resolution electron microscopy methods to identify deleterious imperfections in typical metal-mirror surfaces. Pulsed-laser damage testing of identical surfaces shows systematic changes in the damage resistance of the material that can be correlated directly to defects in the near-surface region. By this method, it is possible to identify trace impurities that are deleterious to high fluence optical components. In addition, one can now specify process parameters that will control not only surface roughness and figure, but will also control laser damage resistance.